Since the flexible printed circuit can be mounted three-dimensionally with high density even within a limited space, their application is expanding. Recently, as electronic devices become more compact and lightweight, the products related to flexible printed circuits are diversifying and the demand for them is increasing. As such products related to FPC, there can be mentioned a flexible copper clad laminate in which a copper foil is bonded to a polyimide film; a flexible printed circuit in which an electronic circuit is formed on the flexible copper clad laminate; a reinforced flexible printed circuit in which the flexible printed circuit is bonded to a reinforcing board; a multilayered board having flexible copper clad laminates or flexible printed circuits layered and bonded with each other; a flexible flat cable (hereinafter often referred to as “FFC”) comprising copper wiring bonded to a base film, and the like. For example, when the flexible copper clad laminate is manufactured, an adhesive is usually used for bonding the polyimide film and the copper foil together.
Further, when the flexible printed circuit is manufactured, a film so-called “coverlay film” is generally used for protecting wired portions. The coverlay film consists of an insulating resin layer and an adhesive layer formed thereon, and a polyimide resin composition is widely used for forming the insulating resin layer. In manufacturing the flexible printed circuits, the coverlay film is bonded to the surface having wiring portions thereon via the adhesive layer by means of, for instance, a heat press. In this instance, the adhesive layer of the coverlay film is required to establish a strong adhesion to both the wiring portions and the base film layer.
In addition, as a printed circuit, a build-up multilayer printed circuit in which a conductor layer and an organic insulator layer are alternately layered on a surface of a substrate is known. When such a multilayer printed circuit is manufactured, a material forming an insulating adhesive layer, so-called a “bonding sheet”, is used for bonding the conductor layer and the organic insulator layer. The insulating adhesive layer is required to have embeddability to the wiring portions and establish a strong adhesion to both materials of the conductor portions forming the circuit (copper and the like) and the organic insulator layer (polyimide resins and the like).
As adhesives for use in the FPC related products, there have been proposed epoxy-based adhesive compositions containing a thermoplastic resin highly reactive with the epoxy resin. For instance, Patent Document 1 discloses an adhesive that is based on an ethylene-acrylate copolymer rubber and an epoxy resin. Furthermore, Patent Document 2 discloses an adhesive that is based on a glycidyl group-containing thermoplastic elastomer and an epoxy resin. Moreover, Patent Document 3 discloses an adhesive that is based on a styrene-maleic acid copolymer and an epoxy resin. Adhesive compositions described in theses references are widely used because they perform fast curing reaction and have excellent adhesiveness by virtue of reactivity of carboxylic groups of the rubber or elastomer components with the epoxy resin.
Furthermore, in the field of mobile communication equipment such as mobile phones and information terminal devices which are now rapidly increasing in demand, higher frequency signals are used to process a huge amount of data at high speed. Accordingly, with the increase in signal speed and signal frequency, the adhesive for use in FPC-related products must satisfy electrical properties in the high frequency region (i.e, low dielectric constant and low dielectric loss tangent). To cope with such demands for electrical properties, for example, Patent Document 4 discloses an epoxy resin composition containing an epoxy resin, a copolymer resin made from, as essential components, an aromatic vinyl compound and maleic anhydride, and a specific phenol compound.